1. Field of the Invention
The present invention relates to a method for manufacturing a pigment dispersion.
2. Description of the Related Art
Agrochemicals such as herbicides and insecticides, drugs such as antitumor drugs, antiallergy drugs, and antiphlogistics, and coloring materials such as inks and toners having colorants are well known as functional materials for dispersion materials containing functional substances.
Among them, quinoline compounds are known as intermediates for agrochemicals and drugs and also as organic EL materials. Further, among them, quinacridone compounds are widely used as coloring materials and application thereof as pigments is known.
A remarkable progress has recently been made in the field of digital printing technology. Electrophotography and ink jet technology are representative examples of digital printing technology, and the presence of such technology as an image forming technology in offices and at home has become more noticeable in recent years.
Among these methods, ink jet technology as a direct recording method features compactness of equipment and low power consumption.
Further, miniaturization of nozzles has rapidly improved the image quality. In one of the methods representing ink jet technology, ink supplied from an ink tank is evaporated and bubbled by heating with a heater located in a nozzle, and the ink is ejected to form an image on a recording medium.
In another example, ink is ejected from a nozzle by inducing oscillations with a piezo element.
Aqueous solutions of dyes are often used as inks to be employed in such methods. As a result, bleeding can sometimes occur in the zones where colors overlap, or a phenomenon called feathering sometimes occurs in the paper fiber direction in the recording locations on the recording medium.
U.S. Pat. No. 5,085,698 discloses using a pigment dispersion ink to reduce bleeding and feathering.
However, in many cases, the pigment dispersion inks can be inferior to dye inks in terms of coloration ability. Thus, because pigment particles can cause light scattering and light reflection, the coloration ability of images formed by pigment inks can be inferior to that of the images obtained with dye inks.
Further, coarse pigment particles can cause nozzle clogging in the ink jet head. Decreasing pigment particles in size has been attempted to improve coloration ability and to reduce nozzle clogging.
Pigments reduced in size to 100 nm or less often demonstrate a low level of light scattering and have a large surface area. As a result, an improvement in coloration ability can be achieved.
Pigment dispersion inks are typically obtained by dispersing a water-insoluble pigment in an aqueous medium. This process may include the step of adding the pigment to an aqueous medium containing a dispersant, and then finely grinding in a dispersion apparatus such as a sand mill or a ball mill using hard beads.
However, in certain instances it may be difficult to obtain a fine and stable pigment dispersion with such a process. Japanese Patent Laid-Open No. 9-176543 discloses a method for obtaining a pigment particle dispersion with a particle size equal to or less than 100 nm by high-speed mill dispersion using beads. With this method a fine dispersion can be reliably obtained, but a large amount of energy may be required for the dispersion process and a complex process may be necessary to separate the dispersion liquid obtained and the beads.
On the other hand, a method for obtaining fine particles of a pigment by dissolving the pigment and then precipitating it again has been suggested. Japanese Patent Laid-Open No. 9-221616 suggests producing fine particles by an acid pasting method in which a pigment is dissolved using sulfuric acid, but such a method may not produce pigments with a particle size of 100 nm or less. Further, Japanese Patent Publication Nos. 4-29707 describes methods for obtaining fine pigment particles by dissolving a pigment in an aprotic polar solvent in the presence of a base and then neutralizing with an acid and precipitating the pigment.
However, because the particle size reduction of the pigment and dispersion stabilization treatment are not typically performed simultaneously, the pigment particles that are initially very small may start aggregating in the course of dispersion, and it is not always easy to obtain a pigment dispersion of essentially nanometric order.
In Japanese Patent Publication Nos. 5-27664, 6-96679, and 6-33353, fine pigment particles are obtained by dissolving a pigment together with a dispersant such as a surfactant or resin in an aprotic polar solvent in the presence of an alkali and then neutralizing with an acid and precipitating the pigment.
The pigments that are dissolved in these methods typically have poor solubility. Therefore, the amount of solvent necessary for dissolving the pigments may be relatively large and thus a high-concentration dispersion may not always be easy to produce. Using a large amount of organic solvent can also increase the production cost and also the cost of waste-water treatment. Further, although the dispersion obtained can be concentrated by vacuum distillation or ultrafiltration of the solvent, significant efforts and time may be required for obtaining a high-concentration dispersion. Therefore, room for improvement still exists.
Further, Japanese Patent Laid-Open No. 2005-307154 discloses a method for producing 2,9-dimethylquinacridone by synthesis using a capillary as a reactor.
In this synthesis reaction, a starting material is mixed with p-toluenesulfonic acid, which is solid at normal temperature, dimethylformamide, and ethylene glycol, the mixed solution is passed through a capillary heated in an oil bath, and 2,9-dimethylquinacridone is synthesized.
However, the p-toluenesulfonic acid, which is solid at normal temperature, can be difficult to pass through a capillary, and thus because a solvent is typically used, it is not always easy to obtain 2,9-dimethylquinacridone at a high concentration. Furthermore, in order to obtain 2,9-dimethylquinacridone using water as a dispersion medium, a phase transition to water may be necessary, and the concentration may be additionally decreased. When the synthesis is performed in the presence of a dispersant, there may also be the risk of the dispersant dissolving at a high temperature under acidic conditions, and the number of suitable dispersants may, therefore, be limited. According, there remains room for improvement.